This section provides background information related to the present disclosure which is not necessarily prior art.
Co-owned U.S. Pat. No. 7,295,731 B2 describes a method for using femtosecond laser radiation for the creation of optical waveguiding devices inside standard optical fibers. The femtosecond laser light is focused inside the optical fiber and the intense light generated in the focal region produces non-linear absorption in the glass fiber and creates permanent refractive index changes. The different refractive index regions can be used to produce waveguide vias for light to travel, thus functionalizing the fiber for applications beyond their original intent. For example, these waveguides can be made to interact with the fiber core and allow light to be coupled from the core to the fiber cladding. This technique can be used to form applications such as local light tapping for power monitoring, cladding waveguides for sensing, periodic structures such as Bragg grating for light filters, and others.
In past realizations of this technique, a fiber was attached directly on top of a photodiode and submerged in an index-matching medium in order to facilitate coupling of light originated in the waveguide tap region. The light coupled from the fiber core to the fiber cladding propagated through said cladding towards the edge of the optical fiber and the index-matching medium frustrated the total internal reflection at the fiber edge and provided for the in-situ measurement of said light with a photosensitive device, mainly a photodiode. One drawback of this invention was the necessity to use the index-matching medium to frustrate the total internal reflection at the edge of the fiber. The cylindrical shape of the fiber also adds a layer of difficulty to the process when the fiber is bonded with index-matching glue to a photosensitive surface. Moreover, the shallow exit angles used for the tap coupler makes it difficult to control aspects such as polarization dependent loss or polarization dependant coupling.
Co-owned U.S. Patent No. 2017/0015585 A1 describes a related implementation where the ultrafast laser writing technique is used in combination with a pre-modification process to achieve coupling of light from the core to the cladding of a photonics crystal fiber (PCF). The fabrication method described in this invention suffers the same drawbacks as the ones described for U.S. Pat. No. 7,295,731 B2 and while the method was able to tap light from the core to the cladding of the PCF it was not simple to control the wavelength dependence of that coupling.